Preparation method of metal nanowire film for liquid crystal phase shifter

Abstract

The invention discloses a preparation method of a metal nanowire thin film for a liquid crystal phase shifter, and aims to solve the problems that the metal nanowire thin film is easy to break, the yield is low, and the process difficulty of connecting the thin film and a grounding copper layer is high. The preparation method comprises the following steps: 1, plating the grounding copper layer of the liquid crystal phase shifter on the glass substrate; 2, electroplating an aluminum layer on the grounding copper layer; 3, taking an oxalic acid solution as an electrolyte, carrying out anodic oxidation treatment twice by adopting a stepped boosting method to form an aluminum oxide template layer, and washing to obtain a substrate with the aluminum oxide template layer; and 4, preparing a 0.1-0.2 mol / L copper sulfate aqueous solution as a copper plating electrolyte, and growing a copper metal nanowire in the aluminum oxide template layer by adopting an electrochemical deposition process. The prepared metal nanowire film is directly and tightly connected with the grounding layer, the problem that an AAO template is prone to fragmentation does not exist, the bonding step is omitted, the yield is about 90% or above, and the yield and production efficiency are improved.

Description

Technical Field

[0001] The present invention relates to a method for preparing a metal nanowire thin film. Background Art

[0002] Phase shifters are key components in phased array systems, which are used to regulate the phase of radio frequency signals and play an important role in phased array systems. Compared with other phase shifters, liquid crystal phase shifters have the ability of continuous tuning, and have the advantages of high linearity, low weight, small occupied space, etc. However, liquid crystal phase shifters also have the disadvantages of large loss and long physical length. To solve these disadvantages of liquid crystal phase shifters, in 2021, researchers at the University of Magdeburg in Germany proposed an optimization scheme (shown in Figure 1 ), which bonds a layer of metal nanowire thin film (NaM) on the ground layer of the liquid crystal phase shifter, can effectively reduce the loss of the liquid crystal phase shifter, and shorten the physical length of the liquid crystal phase shifter. The results were published in the SCI paper "Fast and Miniaturized PhaseShifter with Excellent Figure ofMerit Based on Liquid Crystal andNanowire-Filled Membrane Technologies".

[0003] Different from the commonly called metal nanowire thin film, the metal nanowire thin film used for liquid crystal phase shifters has special requirements and the preparation process is more complex. This metal nanowire thin film is usually composed of anodic aluminum oxide template (AAO) and metal nanowires. As Figure 1 shown, the metal nanowires grow in the holes of the AAO template, with a diameter of 50 - 200 nm and a nanowire spacing of 100 - 200 nm, and they are arranged orderly, parallelly, and vertically.

[0004] The methods for preparing metal nanowire thin films disclosed in the literature "Slow-wave Microstrip Line on Nanowire-based Alumina Membrane" and "Fast and Miniaturized Phase Shifter with Excellent Figure of Merit Based onLiquid Crystal and Nanowire-FilledMembrane Technologies" are as follows:

[0005] 1. Using magnetron sputtering, sputter a layer of metal gold about 200 - 300 nm thick on one side of a specific-sized AAO to act as a seed layer for the growth of metal nanowires;

[0006] 2. Using deionized water as the solvent and copper sulfate and sulfuric acid as the solutes, prepare a copper sulfate solution as the electroplating solution (the copper sulfate concentration is generally 0.1 mol / L, and the pH value of the solution is adjusted to about 2.0);

[0007] 3. Connect the negative electrode of the DC power supply to the gold layer of the AAO after gold plating (generally using the bonding process), and use the electrochemical deposition method to grow copper metal nanowires in the AAO pores;

[0008] 4. After waiting for a certain time, take out the AAO to obtain the required metal nanowire film.

[0009] However, the metal nanowire films prepared by the above process have the following problems when applied to liquid crystal phase shifters:

[0010] a. The success rate of preparing the metal nanowire film is low. The AAO thickness is extremely thin, generally dozens of micrometers. The AAO template is extremely easy to break during the sputtering of the seed layer and the connection with the grounded copper layer, and the yield is low, making it difficult to apply to actual production;

[0011] b. After the metal nanowire film is prepared, it is also necessary to connect the film to the grounded copper layer through the bonding process, which not only has a high process difficulty but also further reduces the yield of the device. Summary of the Invention

[0012] The purpose of the present invention is to solve the problems that the metal nanowire film is easy to break, the yield is low, and the process of connecting the film to the grounded copper layer is difficult, and to provide a preparation method for the metal nanowire film used in liquid crystal phase shifters.

[0013] The preparation method for the metal nanowire film used in liquid crystal phase shifters of the present invention is realized according to the following steps:

[0014] Step 1. Deposit the grounded copper layer of the liquid crystal phase shifter on the glass substrate to obtain a substrate with a grounded copper layer;

[0015] Step 2. Mix aluminum chloride (AlCl 3 ) with 1-ethyl-3-methylimidazolium chloride (EMIC) to obtain an electroplating solution. Connect the aluminum plate to the negative electrode of the DC power supply, and connect the substrate with the grounded copper layer to the positive electrode of the DC power supply, and perform electroplating treatment in the electroplating solution to obtain a glass substrate plated with an aluminum layer;

[0016] Step 3: Under an inert gas environment, anneal the glass substrate plated with an aluminum layer at a temperature of 450°C - 550°C; use a solution with a mass ratio of anhydrous ethanol to water of 1:1 as the solvent, prepare a 0.15 mol / L oxalic acid solution as the electrolyte, connect the negative pole of the DC power supply to the glass substrate plated with the aluminum layer, connect the positive pole of the DC power supply to a graphite rod, and perform a primary anodic oxidation treatment using a stepped voltage increase method. Use 40V as the starting voltage and 110V as the termination voltage. When the voltage rises to 110V, oxidize at a voltage of 110V for 8 - 15 minutes, then add a high-concentration oxalic acid solution to make the oxalic acid concentration in the electrolyte reach 0.2 mol / L, and oxidize at a voltage of 110V for 40 - 60 minutes. After washing, immerse it in a mixed solution of phosphoric acid and chromic acid for soaking treatment to obtain an aluminum layer with the primary oxide film removed;

[0017] Use a solution with a mass ratio of anhydrous ethanol to water of 1:1 as the solvent, prepare a 0.3 mol / L oxalic acid solution as the electrolyte, and perform a secondary anodic oxidation treatment using a stepped voltage increase method. Use 40V as the starting voltage and 110V as the termination voltage. When the voltage rises to 110V, oxidize at a voltage of 110V for 1.5 - 2.5 hours to form an alumina template (AAO) layer, and obtain a substrate with an alumina template layer after washing;

[0018] Step 4: Prepare a 0.1 - 0.2 mol / L copper sulfate aqueous solution, adjust the pH of the copper sulfate aqueous solution to 2 with sulfuric acid to obtain a copper plating electrolyte. Connect the positive pole of the DC power supply to the grounded copper layer on the glass substrate with the AAO template, connect the negative pole of the DC power supply to a copper sheet, and grow copper metal nanowires in the alumina template layer through the copper plating electrolyte using an electrochemical deposition process, controlling the current density to be 10 - 15 mA / cm 2 , thereby obtaining a metal nanowire thin film for a liquid crystal phase shifter;

[0019] Among them, in the stepped voltage increase method in Step 3, the voltage is increased in stages of 20V.

[0020] In the present invention, an alumina template (AAO) layer is directly grown on the grounded copper layer. The grounded copper layer acts as an electrode after the AAO is grown well, and metal nanowires can be directly grown continuously. In the present invention, ethanol is added to the AAO electroplating solution to increase the electroplating voltage and accelerate the oxidation rate of AAO.

[0021] In the preparation method of the metal nanowire thin film for a liquid crystal phase shifter of the present invention, the prepared metal nanowire thin film is directly and tightly connected to the grounding layer, not only does not have the problem that the AAO template is easily broken, but also the bonding step is omitted, and the yield is about 90% or more, greatly improving the yield and production efficiency, making such liquid crystal phase shifters have the effect of actual mass production. Description of the Drawings

[0022] Figure 1 Schematic cross-sectional view of an existing liquid crystal phase shifter and a metal nanowire film (NaM);

[0023] Figure 2 Schematic diagram of electroplating a grounding copper layer in Step 1 of the embodiment;

[0024] Figure 3 Schematic diagram of electroplating and growing a metal aluminum layer on the grounding copper layer in Step 2 of the embodiment;

[0025] Figure 4 Schematic diagram of the anodization process in Step 3 of the embodiment;

[0026] Figure 5 Schematic diagram of growing copper metal nanowires in the alumina template layer by the electrodeposition process in Step 4 of the embodiment;

[0027] Figure 6 Scanning electron micrograph of the metal nanowire film prepared in the embodiment. Detailed implementation manners

[0028] Detailed implementation manner 1: The preparation method of the metal nanowire film for the liquid crystal phase shifter in this implementation manner is realized according to the following steps:

[0029] Step 1: Electroplate the grounding copper layer of the liquid crystal phase shifter on a glass substrate to obtain a substrate with a grounding copper layer;

[0030] Step 2: Mix aluminum chloride (AlCl 3 ) with 1-ethyl-3-methylimidazolium chloride (EMIC) to obtain an electroplating solution. Connect the aluminum plate to the negative electrode of a DC power supply, and connect the substrate with the grounding copper layer to the positive electrode of the DC power supply. Perform electroplating treatment in the electroplating solution to obtain a glass substrate plated with an aluminum layer;

[0031] Step 3: Under an inert gas environment, anneal the glass substrate plated with an aluminum layer at a temperature of 450°C - 550°C; Use a solution with a mass ratio of anhydrous ethanol to water of 1:1 as a solvent to prepare a 0.15 mol / L oxalic acid solution as an electrolyte. Connect the negative electrode of the DC power supply to the glass substrate plated with an aluminum layer, and connect the positive electrode of the DC power supply to a graphite rod. Perform a primary anodization treatment using the step-up voltage method, with a starting voltage of 40V and a termination voltage of 110V. When the voltage rises to 110V, oxidize for 8 - 15 minutes at a voltage of 110V. Then add a high-concentration oxalic acid solution to make the oxalic acid concentration in the electrolyte reach 0.2 mol / L, and oxidize for 40 - 60 minutes at a voltage of 110V. After washing, immerse it in a mixed solution of phosphoric acid and chromic acid for immersion treatment to obtain an aluminum layer with the primary oxide film removed;

[0032] Using a solution with a mass ratio of absolute ethanol to water of 1:1 as the solvent, prepare a 0.3 mol / L oxalic acid solution as the electrolyte, and perform secondary anodic oxidation treatment using the stepped voltage increase method. Starting from a voltage of 40 V and ending at 110 V, when the voltage rises to 110 V, oxidize for 1.5 - 2.5 h at 110 V to form an alumina template (AAO) layer, and after washing, obtain a substrate with an alumina template layer;

[0033] Step 4: Prepare a 0.1 - 0.2 mol / L aqueous copper sulfate solution, adjust the pH of the aqueous copper sulfate solution to 2 with sulfuric acid to obtain a copper plating electrolyte. Connect the positive pole of the DC power supply to the grounded copper layer on the glass substrate with the AAO template, and connect the negative pole of the DC power supply to a copper sheet. Use the electrochemical deposition process to grow copper metal nanowires in the alumina template layer through the copper plating electrolyte, controlling the current density to be 10 - 15 mA / cm 2 , thereby obtaining a metal nanowire film for a liquid crystal phase shifter;

[0034] Among them, in the stepped voltage increase method in Step 3, the voltage is increased in stages of 20 V.

[0035] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that the thickness of the grounded copper layer in Step 1 is 18 - 20 μm.

[0036] Specific Embodiment 3: The difference between this embodiment and Specific Embodiment 1 or 2 is that the molar ratio of aluminum chloride (AlCl 3 ) to 1 - ethyl - 3 - methylimidazolium chloride (EMIC) in Step 2 is 2:1.

[0037] Specific Embodiment 4: The difference between this embodiment and any one of Specific Embodiments 1 to 3 is that the thickness of the metal aluminum layer in Step 2 is 40 - 100 μm.

[0038] Specific Embodiment 5: The difference between this embodiment and any one of Specific Embodiments 1 to 4 is that the current density is controlled to be 6 - 12 mA / cm during the electroplating process in Step 2 2 .

[0039] Specific Embodiment 6: The difference between this embodiment and any one of Specific Embodiments 1 to 4 is that the glass substrate plated with an aluminum layer is annealed at a temperature of 500 °C for 2 h in Step 3.

[0040] Specific Embodiment 7: The difference between this embodiment and any one of Specific Embodiments 1 to 6 is that the temperature of the electrolyte is controlled to be 0 °C using an ice - water bath during the anodic oxidation process in Step 3.

[0041] Embodiment VIII: The difference between this embodiment and any one of Embodiments I - VII is that in Step 3, it is immersed in a mixed solution of phosphoric acid and chromic acid for soaking treatment for 1 - 3 h. The concentration of phosphoric acid in the mixed solution is 6 wt%, and the concentration of chromic acid is 1.8 wt%.

[0042] Embodiment IX: The difference between this embodiment and any one of Embodiments I - VIII is that in Step 3, the thickness of the alumina template (AAO) layer is 40 - 100 μm.

[0043] Embodiment X: The difference between this embodiment and any one of Embodiments I - IX is that the diameter of the oxidation holes on the alumina template layer is 50 - 100 nm.

[0044] Example: The preparation method of the metal nanowire thin film for the liquid crystal phase shifter in this embodiment is realized according to the following steps:

[0045] Step 1: Deposit the grounded copper layer of the liquid crystal phase shifter on the glass substrate. The size of the glass substrate is 20×15 mm to obtain a substrate with a grounded copper layer.

[0046] Step 2: Mix aluminum chloride (AlCl 3 ) and 1 - ethyl - 3 - methylimidazolium chloride (EMIC) with a molar ratio of 2:1 to obtain an electroplating solution. Connect the (pure) aluminum plate to the negative electrode of the DC power supply, and connect the substrate with the grounded copper layer to the positive electrode of the DC power supply. Set the current density to 10 mA / cm 2 , and perform electroplating treatment in the electroplating solution for 10 h to obtain a glass substrate plated with an aluminum layer with a thickness of 80 μm.

[0047] Step 3: Under an argon atmosphere, anneal the glass substrate plated with the aluminum layer at a temperature of 500 °C for 2 h to eliminate the internal stress of the aluminum film. Use ethanol as a solvent (the mass ratio of water to absolute ethanol is 1:1) to prepare a 0.15 mol / L oxalic acid solution as the electrolyte. Connect the negative electrode of the DC power supply to the glass substrate plated with the aluminum layer, and connect the positive electrode of the DC power supply to the graphite rod. Perform primary anodic oxidation treatment using the step - up voltage method. Take 40 V as the starting voltage, 110 V as the termination voltage, and increase the voltage in stages of 20 V. Hold the voltage for 5 min for each 20 V increase. When the voltage rises to 110 V, oxidize at 110 V for 10 min. Then add a high - concentration oxalic acid solution to make the oxalic acid concentration in the electrolyte reach 0.2 mol / L, and then oxidize at 110 V for 50 min. After washing, immerse it in a mixed solution containing 6 wt% phosphoric acid and 1.8 wt% chromic acid and soak at 60 °C for 2 h to obtain an aluminum layer with the primary oxide film removed.

[0048] Using ethanol as the solvent (mass ratio of water to absolute ethanol is 1:1), prepare a 0.3 mol / L oxalic acid solution as the electrolyte, and perform secondary anodic oxidation treatment using the stepwise voltage increase method. Starting from a voltage of 40 V and ending at 110 V, increase the voltage in steps of 20 V, and maintain the voltage for 5 min at each 20 V increase. When the voltage reaches 110 V, oxidize for 2 h at 110 V to form an alumina template (AAO) layer with a thickness of 50 μm. After washing, obtain a substrate with an alumina template layer;

[0049] In this step, during the anodic oxidation treatment process, use an ice-water bath to control the temperature of the electrolyte at 0°;

[0050] Step 4: Prepare a 0.1 mol / L aqueous copper sulfate solution, adjust the pH of the aqueous copper sulfate solution to 2 with sulfuric acid to obtain a copper plating electrolyte. Connect the positive electrode of the DC power supply to the grounded copper layer on the glass substrate with the AAO template, and connect the negative electrode of the DC power supply to a pure copper sheet. Adopt the electrochemical deposition process to deposit copper metal nanowires in the alumina template layer through the copper plating electrolyte, and control the current density at 10 mA / cm 2 , and the deposition time is 1 h, thereby obtaining a metal nanowire thin film for a liquid crystal phase shifter.

[0051] As Figures 2 to 4 shown, in this embodiment, a glass substrate 1 with a grounded copper layer is obtained by electroplating; a glass substrate 2 with a grounded copper layer and an aluminum layer is obtained by electroplating again; a glass substrate 3 with an AAO and a grounded copper layer is obtained by using the anodic oxidation process; finally, metal copper nanowires are grown by the electrochemical deposition process to obtain a glass substrate with a metal nanowire thin film and a grounded copper layer, which can be directly applied to the assembly of a liquid crystal phase shifter. Figure 6 is the SEM image of the surface of the metal nanowire thin film.

[0052] In the present invention, the AAO is directly grown on the grounded copper layer, and the grounded copper layer is used as the seed layer, eliminating the steps of sputtering the seed layer and bonding the metal nanowire thin film to the grounded copper layer. There is no problem of easy fragmentation caused by the overly thin AAO during the processing, and it has the advantages of high yield and simpler processing process.

Claims

1. A method for preparing a metal nanowire film for a liquid crystal phase shifter, characterized in that The method for preparing the metal nanowire film is implemented according to the following steps: Step 1: Plate the grounding copper layer of the liquid crystal phase shifter on a glass substrate to obtain a substrate with a grounding copper layer; Step 2: mixing aluminum chloride and 1-ethyl-3-methylimidazolium chloride to obtain an electroplating solution, connecting an aluminum plate to a negative electrode of a DC power supply, connecting a substrate with a grounded copper layer to a positive electrode of the DC power supply, and performing electroplating in the electroplating solution to obtain a glass substrate plated with an aluminum layer; Step 3, in an inert gas environment, annealing the glass substrate coated with the aluminum layer at a temperature of 450°C-550°C; using a solution mixed with anhydrous ethanol and water in a mass ratio of 1:1 as a solvent, preparing a 0.15 mol / L oxalic acid solution as an electrolyte, connecting the negative electrode of a DC power supply to the glass substrate coated with the aluminum layer, and connecting the positive electrode of the DC power supply to a graphite rod, and performing an anodic oxidation treatment using a step-by-step voltage step-up method, with 40V as a starting voltage and 110V as a termination voltage. When the voltage rises to 110V, oxidize at a voltage of 110V for 8-15min, then add a high-concentration oxalic acid solution to make the oxalic acid concentration in the electrolyte reach 0.2mol / L, oxidize at a voltage of 110V for 40-60min, and after washing, immerse in a mixture of phosphoric acid and chromic acid to obtain an aluminum layer with the primary oxide film removed; A solution of anhydrous ethanol and water in a mass ratio of 1:1 is used as a solvent, and a 0.3 mol / L oxalic acid solution is configured as an electrolyte. A step-up voltage method is used for secondary anodization treatment, with 40 V as a starting voltage and 110 V as a termination voltage. When the voltage rises to 110 V, oxidation is performed at a voltage of 110 V for 1.5-2.5 hours to form an aluminum oxide template layer, and a substrate with an aluminum oxide template layer is obtained after washing. Step 4: Prepare a 0.1-0.2 mol / L copper sulfate aqueous solution, adjust the pH of the copper sulfate aqueous solution to 2 with sulfuric acid to obtain a copper plating electrolyte, connect the positive electrode of the DC power supply to the grounded copper layer on the glass substrate with the AAO template, and connect the negative electrode of the DC power supply to the copper sheet, and use an electrochemical deposition process to grow copper metal nanowires in the alumina template layer through the copper plating electrolyte, and control the current density to 10-15 mA / cm 2 , thereby obtaining a metal nanowire film for a liquid crystal phase shifter; In step 3, the voltage is boosted in stages of 20V in the step-by-step voltage boosting method.

2. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that The thickness of the grounding copper layer in step 1 is 18-20 μm.

3. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that In step 2, the molar ratio of aluminum chloride to 1-ethyl-3-methylimidazolium chloride is 2:

1.

4. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that In step 2, the thickness of the metal aluminum layer is 40-100 μm.

5. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that During the electroplating process in step 2, the current density is controlled to be 6-12 mA / cm 2 .

6. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that In step three, the glass substrate coated with the aluminum layer is annealed at a temperature of 500° C. for 2 hours.

7. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that During the anodizing process in step 3, an ice water bath is used to control the temperature of the electrolyte to 0°.

8. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that In step 3, the mixture is immersed in a mixture of phosphoric acid and chromic acid for 1-3 hours, wherein the concentration of phosphoric acid in the mixture is 6wt%, and the concentration of chromic acid is 1.8wt%.

9. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that In step three, the thickness of the alumina template layer is 40-100 μm.

10. The method for preparing a metal nanowire film for a liquid crystal phase shifter according to claim 1, characterized in that The diameter of the oxidation holes on the alumina template layer is 50-100nm.

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